int duplicate(int* numbers, int numbersLen)
{
    int another[10001] = { 0 };
    for (int i = 0; i < numbersLen; i++)
    {
        if (another[numbers[i]] >= 1)
            return numbers[i];
        else
            another[numbers[i]]++;
    }
    return -1;
    // write code here
}